This invention relates generally to balanced moving armature magnetic transducers, and particularly to means for protecting the moving armature from damage affecting the operating characteristics of a transducer caused by mechanical shock.
In contemporary balanced moving armature magnetic transducers, the element or elements comprising the armature usually function as its own restoring spring, providing mechanical stability and approximate magnetic balance of the armature in its quiescent state. Usually a portion of the armature is surrounded by an electrical signal coil, and functions to convey magnetic signal flux through the coil. Consequently the armature is required to have high magnetic permeability and low coercive force, in addition to providing a restoring spring function.
When materials for armatures are heat treated to develop their magnetic properties, they generally have limited mechanical yield strength. This limits the strength of the armature in its restoring spring function. The resistance to mechanical shock of a magnetic transducer having an armature of such materials is undesirably limited. In particular, a shock from an external source may easily and irreversibly alter the position of the armature by plastic damage, thus destroying its magnetic balance.
The foregoing problem is encountered by the hearing aid or hearing instrument art, in which the sound output generating devices (called receivers) are commonly fabricated using balanced moving armature magnetic transducer technology. In fact, susceptibility to mechanical shock is presently considered the second most likely cause of failure in the field, and failure of the receiver causes failure of the entire hearing aid.
Past efforts have attempted to increase the shock resistance of these transducers by the use of snubbing structures that limit the extent of movement or excursion of the vibratory part of the armature when subjected to shock. For example, U.S. Pat. No. 4,272,654 to Carlson discloses plural discrete ridges or continuous ridges formed of coil encapsulant as snubbing means for the inner arm of a folded armature of the general type disclosed in U.S. Pat. No. 3,515,818 to one of the present applicants. U.S. Pat. Nos. 5,647,013 to Salvage et al and 5,757,947 to Van Halteren et al disclose snubbing means for an armature of the general type disclosed in U.S. Pat. No. 3,617,653 to Tibbetts et al. U.S. Pat. No. 5,647,013 discloses several forms of snubbing means including formations pressed in and away from the plane of the armature body, or blobs of adhesive or other settable material applied to the armature, or a spacer having a restricted opening situated between the coil and the permanent magnet structure, or means for altering the shape of the coil tunnel. U.S. Pat. No. 5,757,947 discloses snubbing means forming a part of the drive pin structure connecting the transducer with a diaphragm, or alternatively a U-shaped element disposed on the side of the magnet elements facing away from the coil. These various snubbing means are provided at designated locations of the armature, but in all cases the direction of snubbing is parallel with the drive pin, i.e., directed to limit the excursion of the armature in the direction of the permanent magnetic flux. In general, this direction is normal to a major plane of the armature.
Analysis of mechanical shocks has found the effects to be complex and dependent on the vector direction of the shock. Instrinsically, a shock of external origin may have any arbitrary direction, as exemplified by a hearing aid inadvertently dropped to the floor. The present applicants have found that the full effect of a given shock upon the subsequent operating properties, and also the subsequent resistance to other shocks, is considerably dependent on the direction of the original shock as well as its magnitude. They have further found that snubbing of the armature in the direction of the drive pin, which may be provided intrinsically by the magnets or pole pieces or by means such as those described in the above-mentioned patents, only partially protects the armature from damage. This is particularly the case for folded armatures of the general type described in U.S. Pat. No. 3,515,818.
It has been discovered that when a basic folded armature transducer of the general type disclosed in U.S. Pat. No. 3,515,818 is shocked in the edgewise direction, i.e. the direction normal to that of the flux field and normal to the direction of extent of its vibratory portion, with or without parallel protective snubbing as described above, significant plastic damage to the armature readily occurred, although in this particular case there appeared to be little shift in the magnetic balance of the transducer. However, the damage to the armature significantly compromised the resistance of the transducer to a shift of magnetic balance under a subsequent shock in a different vector direction. This led to the conclusion that the armature should be snubbed in this edgewise direction of shock, hereinafter referred to as edgewise snubbing.
Analysis was then given to a determination of the degree of edgewise snubbing that would be sufficient to protect the armature not only for edgewise shocks in the direction normal to the magnetic flux and to the direction of extent of the vibratory portion of the armature but also for shocks in other possible vector directions. Analysis determined that for a shock of given magnitude in the edgewise direction, a corresponding edgewise snubbing clearance (the space between the relevant edges of the armature and the snubbing means) could be determined such that the armature would survive elastically. However, it was further found that such degree of edgewise snubbing was not sufficient to protect the armature, and to avoid shift of magnetic balance, under shocks of the same magnitude but in other equally possible vector directions. In fact, it was determined that the edgewise snubbing clearance was required to be reduced by a large factor, for example on the order of three, to provide sufficient practical protection.
Based on the foregoing observations, the features of the present invention include the provision of specific snubber means having a surface or surfaces oriented to limit the edgewise excursions of the armature, i.e. normal to the direction of the permanent magnetic flux and to the direction of extent of the vibratory portion of the armature.
Various means may be provided for this edgewise snubbing, including means limiting the excursions of the armature in the direction normal to both the direction of the magnetic flux and the direction of extent of the vibratory portion of the armature.
The edgewise snubbing means of the invention may take any of several forms including filler pieces or a member having opposed surfaces between which the armature is extended, in either case to provide a desired edgewise snubbing clearance.